Silver salt diffusion process



United States Patent US. or. 96-29 4 Claims Int. Cl. G03c 5/54 ABSTRACT OF THE DISCLOSURE Formation of silver sludge in developer-free alkaline processing baths for repeated silver salt diffusion processing, is retarded by 2,4-dimercaptopyrimidine having in the 6-position a hydrogen or alkyl of up to five carbon atoms.

This invention relates to an improvement in the silver salt diffusion process using processing baths which are free from developer substances and which contain a compound of the mercapto pyrimidin series.

In the conventional form of the silver salt diffusion process, alkaline development baths, usually containing mixtures of hydroquinone and l-phenyl-3-pyrazolidone as developer, are used for the development of the exposed lightsensitive layer. It is also possible, however, for the developer chemicals to be incorporated in the light-sensitive layer or in the transfer layer. In this form of the process, only an aqueous-alkaline solution with a pH of about 11-12, which initiates development, need be used as processing bath. Compared with the conventional developer, a processing bath of this kind has the advantage that it does not contain any developer chemicals which can be oxidized in the presence of air and thus discolour the bath.

Unfortunately, the utility of that modification of the process is limited because small quantities of silver thiosulfate complex enter the bath during processing, where they are reduced to metallic silver, forming a black silver sludge. This sludge soils those parts of the apparatus coming into contact with the bath and, in addition, impairs the white areas of the positive prints.

The problem of sludge formation has affected photographic developers for some time and numerous substances have already been proposed in attempts to eliminate formation of silver sludge. In most instances, the substances proposed are heterocyclic mercapto compounds.

Unfortunately, all these compounds have the disadvantage that they simultaneously interfere with the silver-salt diffusion process, either by undesirably affecting the image tone or by delaying the transfer of the silver halide from the unexposed and undeveloped portions of the silver halide emulsion layer to the trans'fer layer. It is among the objects of the present invention to provide substances for inhibiting silver sludge formation in developer-free processing baths for the silver salt diffusion process.

We now have found that the formation of black silver sludge in developer-free processing baths of the silver salt diffusion process can be avoided by addition to the baths of 2,4-dimercapto-pyrimidine or a derivative thereof. This class of compounds does not have any detrimental effect on the silver salt diffusion process.

Compounds of the following formula have proved 10 be particularly effective:

wherein R represents hydrogen or alkyl with up to five carbon atoms, preferably with up to three carbon atoms, most advantageously methyl.

The alkaline processing baths according to the invention can further contain, for example, the following additives: Alkalies such as sodium hydroxide or sodium salts of boric or phosphoric acid such as Na PO or sodium carbonate.

The processing bath has a pH value of preferably between 11.0 and 12.5. The processing bath further con tains 20 to g./l. of a sulfite, preferably an alkali metal sulfite, particularly sodium sulfite; 0.2 to 2 g./l. of a stabilizer such as KBr or KI; and 0.02 to 0.2 g./l., preferably about 0.07 to 0.09 g./l. of the 2,4-dimercapto-pyrimidine compounds of the present invention.

In addition, substances which influence the image tone or silver salt solvents such as sodium thiosulfate, may also be present in the baths according to the invention. It is of course possible to add to the processing bath other substances which may be required to satisfy the special requirements of the particular copying process.

The processing bath of the present invention is particularly useful for processing light-sensitive materials, which contain a mixture of hydroquinone and 1-phenyl-3-pyrazolidone as developer in the light-sensitive layer. The transfer materials do not have to meet any special requirements either. According to common practice they contain solvents for silver halides preferably sodium thiosulfate, and development nuclei, for example, metallic silver or gold, or sulfides such as nickel sulfide, cadmium sulfide or silver sulfide.

Example 1 A light-sensitive material with a silver halide emulsion layer containing 0.7 g. of hydroquinone and 0.3 g. of 1- 'phenyl-3-pyrazolidone per g. of silver, and 0.25 mol of N=a 'PO '12H O g Na SO (anhydrous) g 40 KBr g 1 2,4-dimercapto-6-methylpyrimidine mg.. Water 1 1 copies are prepared in the manner described using the same solution. Thereafter, the processing bath is pale yellow in color and contains small quantities of a light-colored flaky deposit of the silver salts of 2,4-dimercapto-6- methylpyrim'idine. This deposit does not interfere with processing. It remains light yellow in color even after standing in light for several days. No black silver is deposited.

In a parallel comparison test, which was carried out in the same way except that the processing bath did not contain 2,4-dimercapto-6-methyl-pyrimidine a black silver sludge is found in the bath after only approximately 50 copies have been made. The sludge makes satisfactory processing impossible.

Example 2 A light-sensitive material with a silver halide emulsion layer containing 1.0 g. of hydroquinone and 0.3 g. of 1-phenyl-3-pyrazolidone per g. of silver, and silver halides comprising 0.25 mol of silver chloride and 0.01 mol of silver bromide per kg. of emulsion, is exposed to form an image. The exposed layer is brought into contact with a transfer material which contains silver sulfide development nuclei and approximately 1.6 g. of sodium thiosulfate (Na S O -5H O) per m. and processed in a conventional development apparatus for the silver salt diffusion process with a run-through time of 7 to 8 seconds per sheet (DIN A4). The composition of the processing bath is as follows:

100 copies are made with the same solution in the manner described. Thereafter, the processing bath is pale yellow in color and contains small quantities of a light-colored flaky deposit of the silver salts of 2,4-dimercapto-6- methyl-pyrimidine. This deposit does not interfere with processing. It remains bright yellow in color even after standing in light for several days. No black silver is deposited.

In a parallel comparison test, which was carried out in the same way, except that the processing bath did not contain the pyrimidine compound, the bath contains a black silver sludge after only about 50 copies have been made, making satisfactory processing impossible.

Example 3 Na CO (anhydrous) g 8 NaOH g 1.2 KBr g 1.5

N32S203.5H2O g 2 Na SO g 60 2,4-dimercapto-6-methylpyrimidine g 0.08 Water 1 1 copies are made with the same solution in the manner described. Thereafter, the processing bath is pale yellow in color and contains small quantities of a bright-colored flaky deposit of the silver salts of 2,4-dimercapto-6- methyl-pyrimidine. This deposit does not interfere with processing. It remains bright yellow in color even after standing in light for several days. No black silver is deposited.

In a parallel comparison test which was carried out in the same way except that the processing bath did not contain the pyrimidine compound, the bath contains a black silver sludge after only about 50 copies have been made, making satisfactory processing impossible.

Example 4 Equally good results are obtained with the following processing bath using the same emulsion as described in Example 1:

Na PO (anhydrous) g 50 N21280:; g KBr g 1 Na S O .5H O g 12.5 1 -methyl-2-thiohexahydro pyrimidine g 0.015 Ethylene diamine tetraacetic acid g 1 2,4-dimercapto-6-methyl-pyrimidine g 0.09 Water 1 1 We claim:

1. In the silver salt diffusion process for preparing multiple diffusion transfer prints with a single processing bath that is essentially free of developer, the improvement according to which the formation of silver sludge in the processing bath is retarded by incorporating in that bath an effective amount of a 2,4-dimercaptopyrimidine having in the 6-position a hydrogen or an alkyl of up to 5 carbon atoms.

2. The combination of claim 1 in which the pyrimidine compound is 2,4-dimercapto-6-methyl pyrimidine and its concentration in the bath is from 0.02 to 0.2 gram per liter.

3. In an alkaline silver salt diffusion processing bath that is essentially free of developer, the improvement according to which the formation of silver sludge in repeated processing is retarded by a content of from 0.02 to 0.2 gram per liter of a 2,4-dimercapto-pyrirnidine having in the 6-position a hydrogen or an alkyl of up to 5 carbon atoms.

4. The combination of claim 3 in which the pyrimidine is 2,4-dimercapto-6-methyl-pyrimidine and its concentration is from about 0.07 to about 0.09 gram per liter of bath.

References Cited UNITED STATES PATENTS 2,573,027 10/1951 Kendall et al. 96-77 NORMAN G. TORCHIN, Primary Examiner.

G. COHN, Assistant Examiner. 

